The present invention relates to digital storage oscilloscopes in general and in particular to a digital oscilloscope for displaying waveforms representing component signals of frequency within selectable passbands of an input signal.
The behavior of component signals of frequency within selected frequency bands of a wideband analog signal is often of interest, and spectrum analyzers provide researchers with frequency domain plots of signal amplitudes within a band. However, sometimes researchers wish to view a frequency band of interest as a time domain plot. Oscilloscopes plot signal magnitudes as functions of time, but when a waveform representing an input signal is displayed by a conventional oscilloscope, signal components having frequencies within a particular frequency band of interest are often difficult to observe due to the presence of higher or lower frequency components. An analog bandpass filter is sometimes utilized to remove the higher and lower frequency components from the analog signal before it is applied to the input of an oscilloscope, but many different bandpass filters would be needed in order to separately view a wide range of selectable passbands.
Moreover, when a conventional digital oscilloscope is utilized for displaying a high frequency band of an input signal, the period of time that can be accurately covered by the display is limited. A digital oscilloscope typically samples an input signal at a multiplicity of discrete points, produces a sequence of digital data representative of these points, and stores the sequence in an acquisition memory. The data is then read out of memory and utilized to control display of a waveform on the oscilloscope's screen. The duration of the "time window" covered by the waveform displayed by a digital oscilloscope may be increased by gathering a larger number of points at the same sampling frequency or by gathering the same number of points at a reduced sampling frequency. Since memory resources are limited, increasing the number of sample points is impractical for combinations of long time windows and high sampling frequencies. While reducing sampling frequency to increase window size is more commonly employed, reducing sampling frequency limits the frequencies of the input signal which can be accurately represented. When a bandpass filtered input signal includes components higher in frequency than half the sampling frequency, the waveform display produced therefrom may substantially misrepresent the behavior of the original analog signal due to an "aliasing" effect, e.g., wherein high frequency components in the input signal cause the waveform display to appear to oscillate at a much lower frequency.